Sourdough
Sourdough rode with prospectors into the Klondike Gold Rush of 1898, carried in a pouch around the neck or on a belt. Experienced miners and settlers guarded these pouches fiercely. Inside was a living mix of flour and water, full of wild yeast and bacteria. It raised their bread when other leavening failed them.
The same culture reaches back thousands of years, and it is now thriving again in home kitchens and industrial bakeries alike. How can a paste of flour and water lift a loaf? What makes the bread sour, and why does it stay fresh so long? Where did the tradition begin, and why did bakers once drop it? And do the health claims that surround it hold up?
A loaf excavated in Switzerland has been dated to 3700 BC, and it counts among the oldest sourdough breads ever found. A reference work on food microbiology argues that the practice itself probably began with the start of agriculture in the Fertile Crescent and Egypt. That would put its birth several thousand years before the Swiss loaf. The Roman writer Pliny the Elder later described several different ways of leavening bread with sourdough.
For most of human history, this was simply how bread was made. In the European Middle Ages, barm began to replace it. Barm is the leavening taken from the beer-brewing process. After 1871, purpose-cultured yeast pushed sourdough aside even further.
Louis Pasteur's confirmation of germ theory helped cultured yeasts take hold in English-speaking countries. The same food microbiology reference points out how recent the change is. Counting from 2014, baker's yeast has been in use for less than 150 years. Through the twentieth century, commercial bakeries largely abandoned sourdough.
The tradition was not finished. It was about to cross the Atlantic with French bakers heading for California.
French bakers brought their sourdough techniques to Northern California during the Gold Rush, and San Francisco still counts the bread as part of its culture. The nickname lives on in Sourdough Sam, the mascot of the San Francisco 49ers. The link to the prospectors of 1849 is shakier than the legend suggests. Those gold seekers were more likely to bake with commercial yeast or baking soda. The celebrated San Francisco loaf is a white bread with a pronounced sourness.
The Klondike Gold Rush of 1898 carried the tradition north into the Department of Alaska and Canada's Yukon Territory. In those conditions, yeast and baking soda were much less reliable than a living culture. Cold was not the enemy of a starter, because freezing does not kill it. Excessive heat does.
Old hands in the north came to be called sourdoughs. The word is still applied to any Alaskan or Klondike old-timer. The poet Robert Service helped fix the nickname in Yukon culture with his collection Songs of a Sourdough.
The bacterium named after San Francisco turns out to have a more complicated story than the city's pride suggests.
A week or two in a warm place is how a starter begins. A baker stirs flour and water together, then waits. Wheat flour contains an enzyme called amylase, which breaks starch into the sugars glucose and maltose. The wild yeast can feed on those sugars. With enough time, warmth and fresh additions of flour and water, the mixture settles into a stable culture that makes dough rise.
Bakers have many names for this mixture, including chief, chef, head, mother and sponge. Depending on how much water it holds, a starter may be a pourable batter or a stiff dough.
In San Francisco sourdough, the refreshment ratio is 40 percent of the total weight, roughly 67 percent of the new dough's weight. Refreshment is the baker's word for feeding, and the ratio of old starter to fresh flour and water is critical. Higher ratios bring greater microbial stability and keep the pH low. Below a pH of 4.0, lactobacilli are held back and acid-tolerant yeasts gain the advantage.
A culture ruled by the San Francisco bacterium needs 25-30 degrees Celsius and a feeding every 24 hours for about two weeks, or 336 hours. Gaps longer than three days acidify the dough and can change its microbial ecosystem. Bakeries that rely on sourdough alone refresh several times a day. Hobbyists who bake weekly or less cannot match that pace.
Basil leaves soaked for an hour in room-temperature water are used to seed a traditional Greek sourdough. Other cultures start with unwashed organic grapes, whose skins carry wild yeasts. Water from boiled potatoes is said to boost the bacteria by adding starch. Unbleached, unbromated flour carries more microorganisms than heavily processed flour, and wholemeal flour offers the greatest variety plus extra minerals.
Piped water in most cities is treated with chlorine or chloramine. Some bakers prefer unchlorinated water for their cultures, and bottled water works well. Boiling tap water or leaving it uncovered for 24 hours removes chlorine but not chloramines. Those need an activated carbon filter. A small amount of diastatic malt can give the yeasts maltase and simple sugars to get started.
The reserved portion of a starter can be many years old. One way to keep a culture going is to save some starter and stir in fresh flour and water. Bakers sometimes add sugar, milk, apples or potatoes. The French-style levain technique instead saves a piece of raw bread dough. A portion is pulled aside before shaping, ready to leaven the next batch.
Firm starters, such as the Flemish desem, are sometimes buried in a large container of flour to stop them drying out.
Yeast and lactic acid bacteria trade food inside every sourdough culture. The yeast produces carbon dioxide, which leavens the dough. The bacteria produce lactic acid, which brings the sourness. The bacteria metabolize sugars that the yeast cannot, and the yeast in turn metabolizes the by-products of the bacteria.
The acids made along the way include propionic, formic, acetic and lactic acid. Together they create conditions unfriendly to spoilage organisms and disease-causing microbes. That is why sourdough products stay fresh longer than many other breads. They resist mold without the additives other loaves need.
A dry, cool starter produces a sourer loaf than a wet, warm one. In the dry and cool case, bacteria make more acetic acid relative to lactic acid. In the wet and warm case, the balance tips toward lactic acid. Rye and mixed-rye fermentations call for plenty of lactic acid. Wheat fermentations call for relatively more acetic acid.
A salted wheat-rye dough started from scratch takes about 54 hours at 27 degrees Celsius to stabilize at a pH between 4.4 and 4.6. Salt shapes the outcome. A 4 percent concentration inhibits the San Francisco bacterium, while the yeast Candida milleri can withstand 8 percent.
A Belgian laboratory study followed wheat and spelt doughs refreshed every 24 hours at 30 degrees Celsius. During the first two days, the atypical genera Enterococcus and Lactococcus dominated. From day 2 to day 5, the sourdough specialists Lactobacillus, Pediococcus and Weissella outcompeted them. The yeasts grew more slowly and peaked near days 4-5. By days 5-7, well-adapted strains such as Lactobacillus fermentum and Lactiplantibacillus plantarum had emerged. At their peak, yeasts numbered about 1-10 percent of the lactobacilli.
Fructilactobacillus sanfranciscensis got its name from its discovery in San Francisco starters, but it is not native to that city. It was rarely found in spontaneous sourdoughs, even after repeated feeding cycles. It appeared rapidly in wheat sourdoughs, however, when plant materials were used to start the fermentation.
Ordinary baker's yeast, Saccharomyces cerevisiae, died off in wheat sourdough after two refreshment cycles. It tolerates acetic acid less well than other sourdough yeasts. A continuously maintained, stable sourdough therefore cannot be accidentally contaminated by it.
Glucofructans make up about 1-2 percent of wheat dough, and yeasts free fructose from them. The San Francisco bacterium needs maltose and fructose to produce acetic acid. When it uses up the available fructose, it stops making acetic acid and starts making ethanol.
Four to twelve hours before mixing, a baker feeds the starter so it becomes an active leaven. It is ready when it is bubbly and floats in water. The starter then makes up usually 13-25 percent of the total flour weight in the dough, though formulas vary. A smaller share of cold, unfed starter, called sourdough discard, can also work. At 5-10 percent, fermentation takes longer, and the flavor can improve.
Sourdough rises more slowly than dough made with baker's yeast, because sourdough yeast is less vigorous. In the presence of lactic acid bacteria, though, some sourdough yeasts have been observed to produce twice the gas of baker's yeast. The acidic conditions, together with protein-digesting enzymes from the bacteria, weaken the gluten. The result can be a denser loaf.
Because proofing takes so long, many bakers refrigerate shaped loaves before baking. This is called retardation. It slows the proof and builds a richer flavor. Several no-knead methods exist for those who want to skip the mixing work.
Bread machines are a poor match for traditional starters, since the rise time is too long. Manufacturers offer recipes for their devices, with the starter made in the pan and boosted later with ingredients such as apple cider vinegar. A dough proofed for many hours can instead go into the machine for the baking stage only. Even so, a machine cannot produce the blistered, slashed crust of an oven loaf. That crust usually needs a baking stone and misting of the dough to make steam.
In the oven, a baker makes one or more deep primary cuts so the loaf can expand in a controlled way during oven spring. Shallow slashes are then added purely for decoration.
Type I sourdough is the traditional kind, used as the sole leavening agent. It makes San Francisco sourdough, panettone and rye bread. These doughs are firm, with a pH of 3.8-4.5, and they ferment at 20-30 degrees Celsius.
In Type II sourdoughs, baker's yeast is added to the leavening. Their pH falls below 3.5, and they ferment at 30-50 degrees Celsius for several days without feedings. That heat slows or stops yeast growth. The doughs are more liquid and can be chilled and stored for up to a week. They are pumpable, so they suit continuous bread production systems, and industry adopted them partly because they simplify the multiple-step build of Type I.
Type III sourdoughs are Type II doughs that have been spray-dried or drum-dried. They serve mainly as flavoring agents in industry. Drying-resistant bacteria such as Pediococcus pentosaceus dominate them, and bakers can vary the heat and time of drying to influence caramelization. Makers of non-sourdough bread compensate for the missing cultures with an artificial mix called bread improver.
In northern Europe, bread made from 100 percent rye flour is usually leavened with sourdough rather than baker's yeast. Rye lacks enough gluten to support yeast leavening, so its structure depends on starch and carbohydrates called pentosans. Rye amylase stays active at higher temperatures than the wheat version, which could weaken the loaf as starches break down in the oven. The low pH of a sourdough starter shuts the amylases down, letting the carbohydrates gel and set.
Danish rugbrød is a dense, dark rye bread best known as the base for smørrebrød, the open-faced sandwiches. Mexican birote salado began in Guadalajara as a short French baguette with the yeast swapped for sourdough fermentation. It comes out crunchy outside and soft and savory inside. Amish friendship bread uses a starter with sugar and milk, and it also relies on baking powder and baking soda. German pumpernickel is traditionally made from a sourdough starter, though modern loaves often use commercial yeast.
In Azerbaijan, whole-wheat sourdough flatbreads are traditional. Ethiopia ferments teff flour to make injera. Somalia and Djibouti eat a similar variant, and in Yemen it is called lahoh. In India, idlis and dosa come from a sourdough-style fermentation of rice and black gram.
A review of sourdough's health effects published in 2023 delivered a sobering verdict. Despite wide praise from publications, social media and bakers, it found no sound evidence base for measurable effects on clinical health outcomes. Researchers do point to some plausible mechanisms. Sourdough bread has a relatively low glycemic index. Enzymes active during fermentation break down phytates, which may improve the absorption of some minerals and vitamins, most of them found in the bran. Fermentation also reduces wheat components that may contribute to non-celiac wheat sensitivity and irritable bowel syndrome. Lactic acid bacteria may even improve the texture, aroma and shelf life of gluten-free breads.
During the COVID-19 pandemic, a surge of interest in home baking caused shortages of baker's yeast in stores. Sourdough, which can be propagated at home, filled the gap. The 2010s had already seen fermentation regain ground as a major method in bread production, often alongside baker's yeast.
Today's sourdough community shares starters and tips online, and hobbyists post their loaves on social media. Many bakers name their starters and feed them on elaborate schedules. Some treat the craft as science, measuring flavor and acidity and corresponding with professional microbiologists. Others argue fiercely over whether commercial yeast, grapes or milk belong in a culture. Each starter has a history of its own, and the yeasts floating in the air of any region change it over time.
Sourdough expert Ed Wood isolated millennia-old yeast from an ancient Egyptian bakery near the pyramids of Giza. Carl Griffith's starter, dating to 1847, has been passed down through generations. It is considered exceptional among starters said to be generations old. Professional baker Stacie Kearney writes, "I like the throwback of traditional bread, the things our great grandmothers ate."
In countries with no legal definition of sourdough bread, some loaves sold under the name are leavened with baker's yeast or chemical raising agents, with or without a live starter. The Real Bread Campaign calls such loaves sourfaux, so a label reading sourdough may not guarantee a living culture.
Updated
Common questions
What is sourdough?
Sourdough is a leavening agent containing naturally occurring yeast and lactobacillus bacteria. The word is also used for bread baked with it, while the culture itself is called a starter.
How old is sourdough bread?
One of the oldest known sourdough loaves dates from 3700 BC and was excavated in Switzerland. The practice itself probably began with agriculture in the Fertile Crescent and Egypt, several thousand years earlier.
Why does sourdough taste sour?
Sourdough tastes sour because its lactic acid bacteria produce lactic acid and acetic acid during fermentation. A dry, cool starter makes a sourer loaf than a wet, warm one.
How do you make a sourdough starter?
Mix flour and water and leave the mixture in a warm place for a week or two. The enzyme amylase breaks starch into sugars the wild yeast can use, and regular feeding with fresh flour and water keeps the culture active.
Why was sourdough important in the Klondike Gold Rush?
Yeast and baking soda were much less reliable in the Klondike Gold Rush of 1898, so miners carried starter instead. Freezing does not kill a starter, but excessive heat does, and old-timers came to be called sourdoughs.
What are Type I, Type II and Type III sourdough?
Type I sourdough is the traditional culture used as the sole leavening agent. Type II adds baker's yeast and ferments at 30-50 degrees Celsius, and Type III is dried Type II used as a flavoring agent.
Is sourdough bread healthier than other bread?
Sourdough has a relatively low glycemic index, and its fermentation breaks down phytates. A 2023 review found that a sound evidence base for measurable effects on clinical health outcomes has not been established.
All sources
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